Irradiance influences contents of photosynthetic pigments and proteins in tropical grasses and legumes

Three tropical range grasses (Cenchrus ciliaris, Dichanthium annulatum, and Panicum antidotale) and two range legumes [Macroptilium atropurpureum (siratro) and Stylosanthes hamata (stylo)] were grown under four irradiances, i.e. 100 (1100, control), 75 (I-75), 50 (I-50), and 25 (I-25) % of full sunlight. Accumulation of chlorophyll (Chl) b increased but that of Chi a decreased under low irradiances. The greater accumulation of Chi (a+b) in grasses (particularly in D. annulatum and P. antidotale) under shade predicted their shade adaptability. Among legumes Stylosanthes was more adaptive to the shade than Macroptilium due to its higher accumulation of Chi (a+b). Significant difference in the accumulation of carotenoids under I-25 over I-100 was observed in all the species, which shows the increase in quality of the fodder under limited irradiance. There was a significant decrease in soluble protein content in C ciliaris under I-75, however, no significant difference in protein content was observed under I-50 and I-25, which was also reflected in the SDS pattern with the reduction in content of polypeptides at I-75 and following increase at I-50 and I-25. This was possibly due to reduction of light-induced protein at I-75 and then expression of the stress-induced protein at further reduction of irradiance. Peroxidase activity in C ciliaris increased with the decrease in irradiance and its isozyme pattern showed differences among all treatments, which indicated the role of different peroxidase isoforms at different irradiances.